1. Technical Field of the Invention
This invention can be utilized for a car phone or portable phone services, and for digital mobile communication systems. It can be utilized for mobile communication networks in which there are a plurality of speech coding schemes, and it relates in particular to the techniques employed by control stations for setting and controlling traffic channels (which may include wire links and radio channels).
2. Background of Related Art
VSELP (Vector Sum Excited Linear Prediction) as standardized in Personal Digital Cellular Telecommunication System RCR Standard 27 is an example of a speech coding scheme for use in the transmission of speech signals between mobile stations and switches. However, given that a plurality of speech coding schemes might be adopted in the future, it is possible that a single mobile communication system will have some mobile stations which can select a speech coding scheme from among a plurality of such schemes, and some which can only use a certain specific scheme. It is also possible that a single mobile communication system will have some base stations which can handle a plurality of speech coding schemes, and some which can only handle a certain specific scheme.
If a newly introduced speech coding scheme is superior to an existing scheme, it is desirable to adopt a policy of making preferential use of the new scheme. A prior art example of a mobile communication system will be explained with reference to FIG. 1, which shows the overall configuration of a digital mobile communication system which employs a plurality of speech coding schemes. Mobile station 11 is located in radio zone 1 of base station 21 which supports bit rate .alpha. corresponding to speech coding scheme A and bit rate .beta. corresponding to speech coding scheme B, and base station 21 is served by switch 41. Mobile station 12 is located in radio zone 2 of base station 22 which supports bit rate .beta. corresponding to speech coding scheme B, and base station 22 is served by switch 42. Switches 41 and 42 are in turn served by an upper node. It is assumed that mobile station 11 has codec a for speech coding scheme A and codec b for speech coding scheme B, and that mobile station 12 has only codec b for speech coding scheme B.
A conventional method for matching speech coding schemes is as follows (see Jap. Unexamined Pat. No. 6-6295). When mobile station 11 wants to communicate with mobile station 12, information relating to the speech coding schemes of mobile station 11 and mobile station 12 is acquired by switch 41 from database 51 in which subscriber-related information is stored. This is done on the basis of the subscriber number of called mobile station 12 and the calling subscriber number, which are contained in the SETUP request signal received from calling mobile station 11. In the present case, mobile stations 11 and 12 will then be notified of speech coding scheme B which can be set up at both these mobile stations, whereupon a speech channel is set up.
An alternative method for matching speech coding schemes is as follows (see Jap. Pat. Appl. No. 5-170383, laid open on Jan. 31, 1995 under No. 7-30947. Switch 41 sets up a traffic channel to calling mobile station 11 from which there was a SETUP request, this traffic channel being set up for use by speech coding scheme A having a high preassigned priority. Switch 42 is notified by switch 41 of this speech coding scheme A and attempts to set up a traffic channel to called mobile station 12 for use by this speech coding scheme A. However, if the setting up of a traffic channel to mobile station 12 results in speech coding scheme B, switch 41 on the originating side is notified of the fact that a traffic channel to mobile station 12 has been set up for use by speech coding scheme B, whereupon speech coding scheme A of mobile station 11 is changed to speech coding scheme B.
It has been assumed that the setting up a traffic channel in prior art speech coding negotiation schemes of these sorts is carried out between a switch and a mobile station via a base station. However, in a practical digital mobile communication system a control station is interposed between a switch and a base station, and this control station has the functions of controlling and selecting the traffic channels used by the base station. Setting up a traffic channel for a mobile station which makes a SETUP request is achieved by transferring the required setup information between the control station and the switch, and is begun when the switch notifies the control station that there is a traffic channel assign request. The control station then selects an assignable traffic channel and the switch activates the codec corresponding to the selected traffic channel, thereby setting up the channel. Because there are connection delay problems, the assignment of traffic channels between switches and control stations is carried out in a round trip of request and response.
It is assumed that when there are a plurality of speech coding schemes in a digital mobile communication system, traffic channels will be set in accordance with a prearranged order of priority of these speech coding schemes. However, as noted above, traffic channel selection is performed by a control station, and once a selection has been made it is impossible to change over to another traffic channel until the setting of the selected channel has been completed.
Consequently, each time a traffic channel for use by the speech coding scheme which a switch has instructed a control station to select is to be changed to a traffic channel for use by another speech coding scheme, the switch must re-instruct the control station to select another speech coding scheme, and the resulting delay is significant.
In other words, although the control station is notified of the result of the selection decided by the switch, it is not notified of the prearranged order of priority of the speech coding schemes and other selection conditions, and therefore the decision as regards which speech coding scheme to select cannot be made at the control station.
A mobile station uses a control channel to measure, during standby, the received signal level in the control channel of the radio zone in which it is located, and the received signal level in the control channels of a plurality of radio zones surrounding the radio zone in which it is located, the radio zones being specified from the control station by way of the control channel. Then, when it makes a SETUP request, the mobile station informs the control station of (a) the plurality of control channel numbers of the surrounding radio zones, and (b) the received signal levels of these control channels, this data being sent in pairs. A control station has a peripheral zone selection function which operates as follows. When the control station receives a SETUP request from a mobile station, it stores the plurality of pairs of control channel numbers and received signal levels that are contained within the SETUP request. Then, if it is been impossible for a traffic channel to be selected in the radio zone in which the mobile station is located, the control station deduces, from the stored received signal levels which exceed a preset threshold beyond which selection is possible, and their paired control channel numbers, which surrounding radio zones may serve as peripheral radio zone candidates. This deduction is based on the relation between the preassigned control channel numbers and the peripheral radio zones. The control station then ranks these in order of increasing received signal level and uses them as candidate radio zones. Finally, it assigns a candidate radio zone if it has a free traffic channel.
Accordingly, when, for example, a traffic channel for a mobile station cannot be selected within the radio zone which has received the SETUP request of the mobile station, and there is a peripheral radio zone in which a channel can be selected, and control station X which controls the peripheral radio zone in which channel selection is possible is different from control station Y which controls the radio zone in which the SETUP request of the mobile station is received, control station Y must ask control station X to select a traffic channel. Control station Y must then transfer to control station X the order of priority and other selection conditions relating to the speech coding schemes. A control station also has a handover function whereby the traffic channel in use is changed over to a better quality traffic channel when a lower quality of service obtained by an active mobile station is detected in the radio zone controlled by that control station, or when an active mobile station has moved to another radio zone. Under these circumstances as well, just as in the case of the peripheral zone selection function described above, transfer of selection conditions between control stations is required.
That is, a table which stores the order of priority and other selection conditions relating to the speech coding schemes has to be provided in the control stations, and the contents of this table have to be transferred between control stations. However, none of the mobile communication systems taught in the prior art include any proposals which take this sort of thing into consideration.
The setup of a traffic channel at the beginning of a call is carried out by a control station in accordance with an instruction from a switch, and involves the control station informing the switch of the results of this setup. When a control station receives a traffic channel setup instruction from a switch, it must temporarily store information relating to the selection conditions. Furthermore, with regards to implementing the peripheral zone selection function and the handover function, it is essential for the control stations to temporarily store the selection conditions involved and for this selection information to be transferred between control stations.
Thus, when a traffic channel is selected in an actual digital mobile communication system, a decision regarding which speech coding scheme to select, based on selection conditions, frequently has to be made not only at the switches but also at the control stations. Despite this, the transfer of order of priority and other selection conditions for speech coding schemes is not carried out between control stations in the prior art.
Furthermore, no concrete proposals have yet been made for an optimum way of utilizing speech coding schemes when connecting to another communication network. The other network to which a connection may be made from a mobile station may be the ordinary public switched telephone network or another mobile communication system. In the case of connection to the ordinary public switched telephone network, the speech signal is communicated after converting it from the speech coding scheme characteristic of the mobile communication system to, for example, 64 kbps. In the case of connection to another mobile communication system, sometimes the speech signal will be transmitted after converting it from the speech coding scheme characteristic of the mobile communication system to 64 kbps, and sometimes the connection will involve matching to the same speech coding scheme. Careful thought must therefore also be given to the best method for utilizing speech coding schemes when making multistage connections, as when a speech signal is transmitted after being converted to 64 kbps.